Precipitates present in metal specimens significantly affect material properties such as fatigue properties, hot workability, cold workability, deep drawability, machinability, and electromagnetic properties depending on the shape, size, and distribution thereof.
To cite steel, techniques for enhancing properties of steel products by making use of fine precipitates have been remarkably improved in recent years. The development thereof has caused precipitates to be strictly controlled in production steps. The following contents vary gradually through production steps: the content of an added element present in a metal specimen in the form of a solid solution and the content of the added element present therein in the form of a precipitate. The content of an added element present in a metal specimen in the form of a solid solution is hereinafter referred to as solid solution content. The content of an element present in a metal specimen in the form of a precipitate is hereinafter referred to as precipitation content. For example, an added element combines with nitrogen or carbon during the solidification of molten steel and partly precipitates, whereby precipitates are formed. The precipitates are separated into solid solution fractions and precipitate fractions in a slab-heating step. Some of the solid solution fractions precipitate in a subsequent rolling step because of strain-induced precipitation and the rest of the solid solution fractions precipitates during subsequent transformation. Some of the solid solution fractions sometimes remain in products in the form of solid solutions depending on the cooling rate thereof.
As described above, to produce a good final product, it is necessary to control the change in amount of an added element present in the form of a precipitate or solid solution. For such control, it is essential to accurately analyze the solid solution content or precipitation content of the added element in steel.
Examples of precipitates and/or inclusions in steel materials include borides, phosphides, sulfides, nitrides, carbides, and oxides. Such precipitates and inclusions are hereinafter collectively referred to as precipitates.
Examples of known techniques for determining precipitates include an acid dissolution method, a halogen method, and an electrolytic method as disclosed in The Iron and Steel Institute of Japan, “Handbook of Iron and Steel, 4th edition (CD-ROM),” Vol. 4, Section 2, 3.5 or Agne, “Saishin no Tekkou Joutai Bunseki,” p. 40, 1979. A similarity between the methods disclosed in The Iron and Steel Institute of Japan, “Handbook of Iron and Steel, 4th edition (CD-ROM),” Vol. 4, Section 2, 3.5 or Agne, “Saishin no Tekkou Joutai Bunseki,” p. 40, 1979 is that a matrix is chemically dissolved in an extraction liquid and target precipitates are collected from the extraction liquid and then analyzed. The processes are selectively used depending on the type of the target precipitates. The acid dissolution and halogen methods cause the dissolution of carbides and sulfides and therefore are mostly used to selectively extract only oxide inclusions. An electrolytic method using a non-aqueous electrolytic solution is used to extract almost all inclusions from a steel material with no loss.
There is no known technique for directly analyzing the solid solution content of an added element. A similar technique is one disclosed in JIS G 1257 “Methods for atomic absorption spectrochemical analysis of Iron and Steel,” 8.3.1.3, p. 70, 1997. JIS G 1257 “Methods for atomic absorption spectrochemical analysis of Iron and Steel,” 8.3.1.3, p. 70, 1997 discloses a method for analyzing a filtrate obtained by the acid dissolution method, that is, a method for analyzing aluminum in acid-dissolved steel. In that method, the analysis result is the sum of the content of dissolved aluminum and the content of precipitated aluminum, which is acid-soluble, and therefore the solid solution content of aluminum cannot be accurately determined. Thus, to analyze a solid solution component only, an indirect method (hereinafter referred to as the indirect method) is probably used. In the indirect method, the precipitation content of a target component analyzed by the electrolytic method is subtracted from the content of the target component analyzed by, for example, a method for spark discharge atomic emission spectrometric analysis (JIS G 1253:2002) as given by the following equation:[sol. A]=[total A]−[pre. A]  (1)wherein [sol. A] represents the solid solution content of Element A, [total A] represents the content of Element A, and [pre. A] represents the precipitation content of Element A.
The precision of the analysis of each content in Equation (1) can be discussed using the standard deviation a of the analysis result of the content. The standard deviation σsol. A of the analysis result of the solid solution content [sol. A] is statistically given by the formula (σtotal A2+σpre. A2)1/2 from the standard deviation σtotal A of the analysis result of the content [total A] and the standard deviation σpre. A of the analysis result of the precipitation content [pre. A]. Therefore, when at least one of the standard deviation σtotal A of the content and the standard deviation σpre. A of the precipitation content is large, the analytical precision of the solid solution content [sol. A] determined by the indirect method is inevitably low. In general, the magnitude of error in an analysis result is proportional to the content of an element to be analyzed; hence, there is a problem in that the indirect method is low in analytical precision when the precipitation content is large.
Problems involved in analyzing fine precipitates in steel by the indirect method will now be described. Techniques for producing steel materials have been remarkably improved in recent years. Hence, the size of precipitates in steels has been reduced to several nanometers. In the case where fine precipitates are extracted from steel and then collected by filtration, the precipitation content has a negative error because some of the fine precipitates are unavoidably uncollected. In contrast, the solid solution content determined by the indirect method has a positive error. For a specimen containing fine precipitates, a filter with a pore size less than the size of precipitates to be analyzed is inevitably used. However, there is no filter capable of completely separating precipitate particles with a size of several nanometers from a liquid. If a filter having sub-nanometer-sized pores was developed, it is clear that a significant reduction in filtration rate causes a serious reduction in operability. These are not problems unique to steel. Japan Institute of Metals, “Materia Japan,” Vol. 45, No. 1, p. 52, 2006 describes that a precipitate fraction of a target element cannot be separated from a solid solution fraction thereof in the case where fine precipitates are collected from a copper alloy by filtration. Therefore, it is probably difficult to apply the indirect method to a specimen containing fine precipitates.
There is no known technique capable of directly analyzing a precipitate component in a metal specimen probably because of the above problems.
Japanese Unexamined Patent Application Publication No. 59-58342 discloses a method for analyzing a solution by emission spectroscopy. In that method, the solution is prepared from a metal specimen and the content of each element is determined from the spectrum of the element, obtained by analyzing the solution, using a correction formula. The method disclosed in Japanese Unexamined Patent Application Publication No. 59-58342 is based on the following two assumptions:                (1) the sum of the contents of all elements contained in the metal specimen is equal to 100% by mass and        (2) there is a certain relationship between the spectrum intensity ratio of two elements measured at the same time and the content ratio of the two elements.        
Since Assumptions (1) and (2) need to be simultaneously satisfied, almost all elements other than a target element in the solution need to be measured. Assumption (1) requires that the composition of the metal specimen needs to be substantially the same as the composition of the solution. Hence, that method cannot be used when the solution, which is prepared from the metal specimen, is different in composition from the metal specimen.
It could therefore be helpful to provide a method for readily, quickly, and directly determining the solid solution content of a target element in a metal specimen.
To achieve the object, a method for directly analyzing a solid solution component only has been investigated in such a manner that a matrix is dissolved in a non-aqueous electrolytic solution by an electrolytic method and the non-aqueous electrolytic solution is then analyzed.
For a specimen containing fine precipitates, there is the problem in that, even if a filtrate obtained by removing an undissolved residue from a non-aqueous electrolytic solution by filtration is used, the filtrate is contaminated with the fine precipitates and, therefore, the analysis result of the solid solution content has a positive error.
We thus provide:                [1] A method for analyzing a metal specimen comprises:                    an electrolysis step of electrolyzing a metal specimen containing a reference element and a target element in an electrolytic solution;            a sampling step of sampling a portion of the electrolytic solution;            an analysis step of analyzing the sampled electrolytic solution;            a concentration ratio-calculating step of calculating the concentration ratio of the target element to the reference element in the electrolytic solution on the basis of the analysis results; and            a content-calculating step of calculating the content of the target element present in the form of a solid solution by multiplying the obtained concentration ratio by the content of the reference element in the metal specimen.                        [2] In the metal specimen-analyzing method according to Item [1], the sampling step may include sampling a portion of the electrolytic solution during electrolysis.        [3] In the metal specimen-analyzing method according to item [1], the sampling step may include sampling a portion of the electrolytic solution after electrolysis.        [4] In the metal specimen-analyzing method according to Item [1], the sampling step may include removing the rest of the metal specimen from the electrolytic solution after electrolysis and then sampling a portion of the electrolytic solution.        [5] In the metal specimen-analyzing method according to Item [1], the sampling step may include filtering the electrolytic solution and then sampling a portion of the filtered electrolytic solution.        [6] In the metal specimen-analyzing method according to Item [1], the sampling step may include the electrolytic solution of 5 ml or less.        [7] In the metal specimen-analyzing method according to Item [1], the analysis step may include mixing the sampled electrolytic solution with an aqueous solution of a chelating agent to convert the reference element and the target element into water-soluble chelates, and then performing analysis.        [8] In the metal specimen-analyzing method according to Item [7], the chelating agent aqueous solution may be an aqueous solution of ethylenediaminetetraacetic acid.        [9] The metal specimen-analyzing method according to Item [1] may further include the step of dialyzing the sampled electrolytic solution through a semipermeable membrane to remove precipitates and inclusions.        [10] In the metal specimen-analyzing method according to Item [1], the analysis step may include:                    an aqueous solution-preparing step of preparing an aqueous solution by adding a chelating agent to the sampled electrolytic solution;            a checking step of checking that the aqueous solution contains no fine particles; and            an aqueous solution-analyzing step of analyzing the checked aqueous solution.                        [11] In the metal specimen-analyzing method according to Item [10], the checking step may include introducing the aqueous solution into a high-sensitivity analyzer and then checking the aqueous solution on the basis of the stability of the intensity of a signal obtained by time-resolved photometry.        [12] In the metal specimen-analyzing method according to Item [1], the reference element may be an element which forms no precipitate or inclusion.        [13] In the metal specimen-analyzing method according to Item [1], the metal specimen may be steel and the reference element may be iron.        [14] In the metal specimen-analyzing method according to Item [1], the metal specimen may be stainless steel and the reference element may be chromium or nickel.        [15] In the metal specimen-analyzing method according to Item [1], the metal specimen may be a copper alloy and the reference element may be copper.        
Our methods are capable of accurately and directly determining the solid solution content of a target element in a metal specimen. The method is applicable to a metal specimen containing nanometer- or sub-nanometer-sized fine precipitates, is insensitive to the size and amount of precipitates, and is applicable to any metal specimens.
A portion of an electrolytic solution is sampled and then analyzed and the concentration ratio of a target element to a reference element in the electrolytic solution is used to determine the solid solution content of the target element. Hence, the solid solution content of the target element can be readily and quickly determined. Furthermore, the following problems are solved: environmental problems caused by the use of non-aqueous electrolytic solutions and problems involved in quantitative reproducibility due to the volatility thereof.
The solid solution content of an element in a metal specimen is an important evaluation factor for promoting the development of metal products or an important factor for assuring product quality. Therefore, our methods are industrially advantageous because the solid solution content of a target element in a metal specimen can be directly and accurately determined.